1. Field of the Invention
The present invention relates to a radiation imaging apparatus, a method for driving the same, and a radiation imaging system.
2. Description of the Related Art
A radiation imaging apparatus reads out, from a sensor array, a signal corresponding to the amount of charges accumulated in accordance with the irradiation of radiation and forms a radiation image based on this signal. Japanese Patent Laid-Open No. 9-140691 discloses a method for forming a radiation image by periodically reading out signals from a sensor array in the interval from before the irradiation of radiation to after the end of the irradiation, storing the respective obtained signals in a frame memory, and adding/averaging (or adding) the signals. Since this method periodically reads out signals from the sensor array, it is not necessary to synchronize the apparatus with the irradiation timing of radiation when performing radiation imaging. This method however leads to an increase in power consumption because of the necessity to always maintain the circuit in an active state, and requires a large-capacity memory for storing periodically readout signals in the frame memory.
In contrast to this, recently, there is available a technique of detecting the irradiation of radiation first and then automatically starting to read out signals from a sensor array in response to the detection. For example, Japanese Patent Laid-Open No. 2010-268171 discloses the arrangement of a radiation imaging apparatus including a detection means for detecting radiation for radiation imaging. This detection means detects radiation by detecting a current in a bias line connected to each sensor of the sensor array. This arrangement makes it possible to perform the initializing operation of initializing each sensor of the sensor array before, for example, the detection of radiation, and start the readout operation of reading out signals from the sensor array in response to the detection of radiation.
On the other hand, when starting irradiation of radiation in the middle of initializing operation, some of the signals which should be acquired are lost. In contrast to this, Japanese Patent Laid-Open No. 2002-181942 discloses a technique of correcting the signal obtained by readout operation by holding the signal read out from each sensor by initializing operation and compensating for the component lost due to the irradiation of radiation in the middle of initializing operation by using the held signal.
When starting readout operation during the irradiation of radiation, some of the signals which should be acquired are lost as in the above case. For example, under imaging conditions configured to irradiate weak radiation for long period of time, a detecting unit configured to detect radiation may erroneously detect the end of the irradiation of radiation or a timeout may occur without detection of the end of the irradiation. In such a case, a region where readout operation has started before the end of the irradiation of radiation differs in the amount of accumulated charges from a region where readout operation has started after the end of the irradiation of radiation. This makes it impossible to obtain a proper radiation image.